[EFL] Add ewk_intro.h to describe main page of doxygen
https://bugs.webkit.org/show_bug.cgi?id=136013

@cherry-pick

Reviewed by Gyuyoung Kim.

* PlatformEfl.cmake:
* UIProcess/API/efl/EWebKit2.h.in:
* UIProcess/API/efl/ewk_intro.h: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172685 268f45cc-cd09-0410-ab3c-d52691b4dbfc